The present invention relates to a copier, a printer, a facsimile and an image forming apparatus employing the electro-photographic method and having the functions mentioned in the above, and specifically relates to a color image forming apparatus that is provided with an intermediate transfer member on which a plurality of unicolor images are overlapped on each other so as to form a full color toner image.
Well known has been a color image forming apparatus that employs the electro-photographic method in which a toner image formed on an image bearing member serving as a photoreceptor element is primarily transferred onto the intermediate transfer member, and then, the toner image transferred onto the intermediate transfer member is secondarily transferred onto a transfer material. In such a color image forming apparatus, unicolor toner images sequentially formed on the plurality of image bearing members and charged in a predetermined polarity are transferred onto the intermediate transfer member by employing the electrostatic actions in such a manner that the unicolor toner images are superimposed on each other, and then, the full color toner image formed on the intermediate transfer member is further transferred onto the transfer material by employing an electrostatic action.
Since it is possible for the color image forming apparatus employing the intermediate transfer member to superimpose the unicolor toner images formed on a single or a plurality of image bearing member(s) on the intermediate transfer member, such a color image forming apparatus has been widely applied as an color image forming apparatus to form full color images on the transfer material. In this type of color image forming apparatus, the unicolor toner images formed on a single or a plurality of image bearing member(s) are transferred onto the intermediate transfer member in such a manner that the unicolor toner images overlap each other, and then, the overlapping toner image formed on the intermediate transfer member is further transferred onto the transfer material by employing the electrostatic action.
Since an electrostatic charge amount per one toner particle is substantially uniform among plural toner particles, an electric potential of the toner layer residing on the intermediate transfer member varies depending on an amount of toner attached within a predetermined area. Accordingly, in this color image forming apparatus, within the overall toner image formed on the intermediate transfer member, an electrostatic charge potential of a partial area on which plural unicolor toner images overlap each other is greater than that of other partial areas on which only a single unicolor toner image exists. Further, for instance, when the overall toner image formed on the intermediate transfer member includes both a solid color area and a halftone color area, an electrostatic charge potential of the solid color area is greater than that of the halftone color area.
Further, variation of the electrostatic charge potential within the overall toner image after passing through a primary transferring section, at which the unicolor toner image is transferred from the image bearing member to the intermediate transfer member, sometimes would occur due to environmental factors.
When the electric potential of the toner image residing on the intermediate transfer member widely varies as mentioned in the above, various areas whose transferring characteristics are different from each other, coexist in the same toner image. Therefore, attempting to transfer such image areas, whose transferring characteristics differ from each other, onto the transfer material under the same transferring condition, various kinds of image defects are liable to occur at the time of the secondary transferring operation from the intermediate transfer member to the transfer material.
In recent years, the color imaging trend has proliferated in the field of copiers, printers, facsimiles and compound image forming apparatus combining the functions of the abovementioned apparatuses, and, as for the transferring process, the demands for producing high-quality images have been getting larger than ever, due to the progressing trend of employing the polymer toner and the micro particle toner. In addition, the trend of increased production of the image forming apparatuses has also progressed. To obtain excellent images while meeting such trends, it is necessary to compensate for the electric potential of the toner layer residing on the intermediate transfer member, which is liable to vary depending on a number of primary transferring operations and the environmental factors, so as to make it uniform over the toner layer and to improve the secondary transferring efficiency.
Patent Document 1 (Tokkaihei 10-274892, Japanese Non-Examined Patent Publication) sets forth the apparatus that is provided with a discharge section for making the electrostatic charge potential of the toner layer residing on the intermediate transfer member uniform, before transferring the toner image onto the transfer material.
Patent Document 2 (Tokkaihei 11-143255, Japanese Non-Examined Patent Publication) sets forth the apparatus in which the potential difference controlling section controls the power source of the discharge section as well as other power source of the secondary transferring section, so that the potential difference between the electric potential of the toner layer is made uniform by the discharge section, and the other electric potential of the secondary transferring section is kept substantially constant.
As mentioned in the foregoing, in the color image forming apparatus in which the unicolor toner images borne by a single or a plurality of image bearing member(s) are transferred onto the intermediate transfer member by the primary transferring section(s) in such a manner that the unicolor toner images overlap with each other, and then, the overlapping toner image borne by the intermediate transfer member is secondarily transferred from the intermediate transfer member onto the transfer material, since an amount of toner charge of the toner image formed on the intermediate transfer member is liable to vary depending on a number of primary transferring operations and ambient environmental factors, various kinds of image defects are liable to occur at the time of the secondary transferring operation from the intermediate transfer member to the transfer material.
The color image forming apparatus set forth in Patent Documents 1 and 2 (Tokkaihei 10-274892 and Tokkaihei 11-143255, both being Japanese Non-Examined Patent Publication) compensates for an electrostatic charge amount of toner upstream side of the secondary transferring section by employing a scorotron charger. In other words, this is a technology for evening the electrostatic charge amount of toner, primarily transferred onto the intermediate transfer member, by employing the corona discharging phenomenon, such as the AC discharging action, the DC discharging action, etc.
In the color image forming apparatus set forth in Patent Documents 1 and 2, the toner image is further charged in the operation for compensating for the electrostatic charge amount of toner. Namely, the toner image already charged at a negative polarity is further charged by applying the corona discharging operation caused by an electric current of negative polarity. According to this method, since the overall electrostatic charge potential is getting higher, the variations of the electrostatic charge amount of toner become relatively unnoticeable.
However, when charging the overall toner image in the manner as mentioned in the above, since the electrostatic charge potential of the toner image to be secondarily transferred is also getting higher, the transferring bias voltage, necessary for transferring the toner image, should also be higher than that in normal conditions. As a result, the operation for controlling the transferring bias voltage becomes difficult, and the problems of image defects, such as electrical destruction of the toner image, etc., would arise. Further, since the electronic current necessary for the transferring operation is getting greater, the transferring efficiency is also getting worse. Still further, although it is possible to lower the electrostatic charge amount of the overall toner image by developing the latent image with toner whose electrostatic charge amount is lowered in advance, there would arise another problem that the toner are scattered around the developing section.
To cope with the abovementioned problems, the present inventors have invented a novel process for making the electrostatic charge amount of the overall toner image uniform in such a manner that a part of the toner image having a higher electrostatic charge amount is lowered by employing a dischargeable bias voltage, instead of further charging the toner image.
However, when the total electrostatic charge amount of the overlapping toner image part, namely, the higher potential part, is lowered in order to make the electrostatic charge amount uniform over the toner image, the electrostatic charge amount of the lower potential part (on which a smaller amount of toner exists) is also discharged at the same time, possibly resulting in occurrence of image defects, such as roughness of the halftone image portions, etc.
For instance, when simply employing a strong discharging operation, although the sufficient discharging effect for the portions carrying large amount of toner can be achieved, the electric potential at the portions carrying the small amount of toner are also lowered. On the contrary, when the discharging operation is weak, it is impossible to decrease the potential value of potions carrying the large amount of toner to a desired value, resulting in a difficulty of satisfying both of the abovementioned factors at the same time. To prevent such the deficiency, a controlling operation employing a grid electrode becomes necessary.
On the other hand, for instance, when the discharging operation of the positive polarity is applied to the toner layer, having the negative electric charge, so as to lower the toner layer potential in the discharging section before secondary transfer, the polarity of a certain part of the toner layer would be reversed. At this time, by applying the negative electric potential to the grid electrode so as to control the electric potential of the toner layer, the positively charged toner are attracted to the grid electrode and attached to the grid electrode, due to the electric field generated between the grid electrode and the intermediate transfer member. When the grid electrode is contaminated with toner, the functional efficiency of the grid electrode is also deteriorated. This results in the degradation of the controllability for the electric potential of the toner layer, a decrease of the electric potential at the toner carrying areas and the occurrence of image defects, such as roughness of the halftone image potions, etc. Accordingly, it has been difficult to obtain stable controllability, due to the degradation of the durability.